Displacement sensors, such as microphones, accelerometers, and pressure sensors, are well-known. Displacement sensors based on capacitive, impedance, and optical measurements have been developed. Optical displacement sensors are particularly attractive as they overcome many of the limitations of capacitive and impedance measurement techniques, such as low sensitivity, the need for high voltage biasing, poor electrical isolation, or response nonlinearities.
Many optical displacement sensors known in the prior art operate by detecting light reflected by an optical element that changes its reflectivity in response to a pressure differential, sound, vibration, etc. An optically resonant cavity, such as a Fabry-Perot interferometer arrangement, has often been used as such an optical element. An optically resonant cavity has a reflectivity that depends on the spacing between two parallel partially-reflective surfaces. In order to form an optically resonant cavity that is sensitive to an environmental stimulus, such as sound, acceleration, etc., one surface of the optically resonant cavity is a movable surface. When the movable surface moves in response to the environmental stimulus, the reflectivity of the cavity is changed. The intensity of the detected light changes as well, therefore, thereby resulting in an electrical signal based on the incident acoustic energy of the sound.
In many optical displacement sensors, a source is used to provide light that is directed toward the optically resonant cavity. Since in most cases, the source is a coherent light source, it is important to keep light from being reflected back from the cavity into the source. Such back reflections can cause source instability, noise, and negatively impact reliability.
In addition, the alignment between such a source, an optically resonant cavity, and additional optical components typically must be done with high precision that has to be maintained throughout the life of the displacement sensor. The need to attain and maintain tight alignment tolerances between multiple optical components can lead to high cost of such systems.
A packaging approach that provides good back reflection suppression and also eases alignment tolerances is, therefore, highly desirable.